The Antarctic Ocean Zones

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The Antarctic Ocean Zones

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The Antarctic waters are those that lie south of the Antarctic Polar Front or Antarctic Convergence, in an area also known as the Southern Ocean. The region is characterised by cold surface waters (< 4 C), which are generally nutrient poor but, where the waters meet the continental shelf, nutrients are liberated and cause local areas of enhanced productivity. Close to the Antarctic continent the temperatures are even cooler and the ocean surface freezes each winter to form sea-ice.

Epipelagic faunaThe epipelagic zone is the upper sunlit zone of the oceans, typically extending from the surface to around 200 m. This is the productive zone of the ocean, in which tiny plants, the phytoplankton, use the energy of the sun to grow and reproduce. Production in the epipelagic zone feeds the oceans, either by material falling from the surface layer or by deeper living species migrating to the epipelagic zone to feed.

Antarctic krill (Euphausia superba) is widely regarded as the most important species found in this zone. Antarctic krill is a small shrimp-like animal that reaches around 50 mm in length. It is highly abundant, particularly in the area of the Antarctic Peninsula and Scotia Sea, forming enormous swarms which can be miles in length and a hundred metres deep. Krill are regarded as the key species in the Southern Ocean food-web, consuming phytoplankton and providing the principal prey for many species of penguin, whale and seal.

Antarctic silverfish (Pleuragramma antarcticum) are another important member of the epipelagic fauna, but are restricted to the cold (0 to -2 C) surface waters close to the Antarctic continent. In order to survive at such cold temperatures, the blood of Antarctic silverfish contains special anti-freeze proteins. Antarctic silverfish reach a maximum size of 25 cm, and whilst the younger fish are limited to the epipelagic layer, adults can be found as deep as 700 m. Antarctic silverfish are important prey for emperor and Adélie penguins, Antarctic toothfish and Weddell seals.

The Weddell seal (Leptonychotes weddellii) is a large seal (up to 600 kgs) that is named after the British explorer and sealer, James Weddell. Weddell seals are the most southerly of all mammals and breeds on fast ice, close to the Antarctic continent. Adults are brownish, with a lighter mottled underside. These psychrophilic seals dive to depths of 500 m or more and can stay under water for over 80 minutes whilst they forage on fish and squid.

Mesopelagic faunaAlso known as the twilight zone, the mesopelagic layer extends from the lower limit of the epipelagic layer to 1000 m. Only a limited amount of light reaches the mesopelagic layer and many of the animals produce their own light using a process called bioluminescence. Bioluminescence can be used to attract prey, attract mates, as a means of distracting predators or as a form of camouflage. Some mesopelagic animals will venture into the surface at night to feed, but return to the relative safety of the darker, deeper mesopelagic zone during day.

The Antarctic lantern fish (Electrona antarctica) is one of the most abundant fish in the mesopelagic layer of the Southern Ocean. Like other lantern fish, this silvery fish, which grows to around 120 mm, has a row of light organs on the underside, which are thought to camouflage the fish when viewed from below by possible predators. Male and female lantern fish also have different lights on the tail region, which are important for identifying potential partners. These abundant fish feed on small crustaceans and are themselves the prey of penguins and seals.

The glacial squid (Galiteuthis glacialis) is one of the most abundant and widely distributed squid species in the Southern Ocean, and reaches a maximum size of around 50 cm. Smaller squid are found in depths of 200-400 m, whilst larger adults occur deeper. Glacial squid predate on planktonic crustacea and are consumed by Ross seals, fish and albatross. How the deep-living squid are consumed by albatross is a bit of a mystery, but it is thought that female glacial squid float to the surface after their single spawning event and are then susceptible to feeding albatross.

The pram bug (Phronima sp.) is a strange type of amphipod crustacean that is found in the mesopelagic layers of the world’s oceans and are said to have been the inspiration for film Alien. Female pram bugs use their powerful claws to turn gelatinous salps into mobile nurseries for their eggs and young.

Bathypelagic faunaThe bathypelagic (or midnight) zone, which extends from 1,000 m down to 4,000 m, is completely dark except for the bioluminescence produced by many of the animals. The name bathypelagic is derived from the Greek bathys meaning deep. The fauna of the bathypelagic zone is not unique to the Southern Ocean, with many species also known to occur further north.

The anglerfish or dreamer (Oneirodes notius) is a rather fearsome looking ambush predator that uses a modified fin as an illuminated fishing rod to lure prey. Unwitting fish that are attracted to the light are quickly snaffled by the large mouth, which is armed with sharp teeth. The dark colour of the fish ensures it is invisible to both predators and prey, but it is known to be consumed by Antarctic toothfish.

The colossal squid (Mesonychoteuthis hamiltoni) is one of the giants of the deep-sea, and the largest living invertebrate, reaching over 10 m in length and up to 750 kgs in weight. The eight arms and two tentacles of the colossal squid are equipped with suckers and rotating hooks that enable it to grasp their fish prey and, possibly, battle with predators, such as the sperm whale. Many sperm whales carry scars that are believed to be caused by the hooks of the colossal squid.

The scaly dragonfish (Stomias boa boa) is a slender fish, with a large mouth and an impressive set of teeth. The scaly dragonfish is widely distributed in the world’s oceans, inhabiting the bathypelagic zone during the day, but venturing into the mesopelagic at night to find its prey. It uses its barbell to attract fish prey and has a row of lights on the underside to provide camouflage.

Abyssopelagic faunaThe abyssopelagic zone, which extends from 4,000 m to the sea-floor, is one of the most inaccessible and hence least known regions on the planet. The term abysso is derived from the Greek abyssos, meaning bottomless. This sparsely populated zone has little energy input and the fauna includes some strange creatures that have adapted to a lightless and low energy environment.

The dogtooth grenadier (Cynomacrurus piriei) is a pelagic rattail that reaches 50 cm total length. The rattails are a group of deep-sea fish that take their name from the long, tapered, rat-like tail. Most are associated with the sea-floor, but the dogtooth grenadier is an exception being pelagic. Although included in the abyssopelagic fauna, it is also found in the bathyepelagic, which has been subject to much greater sampling effort.

The deep-sea alarm (or crown) jellyfish (Atolla wyvillei) has a deep red disc surrounded by 20 normal tentacles and one enlarged one. The alarm jellyfish gets its name from the bioluminescent flashes it emits when startled or attacked by predators. These are thought to be a “burglar alarm’, intended to attract bigger predators to whatever is attacking the jellyfish, allowing the jellyfish to escape predation.

The blind or dumbo octopus (Cirrothauma murrayi) is one of the deep-living “dumbo octopus” that have large fins and arms covered with long hair-like cirri. The blind octopus reaches a metre in length and has been caught in nets or on camera from as deep as 5000 m. Although also known as the blind octopus, Cirrothauma does have vestigial eyes, but they lack lens and are simply light sensitive organs.